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Selective Transfer of Rotationally Commensurate MoS <sub/>2</sub> from an Epitaxially Grown van der Waals Heterostructure
摘要： Large-scale synthesis of high quality two-dimensional (2D) semiconductors are critical for their incorporation in emerging electronic and optoelectronic technologies. In particular, chemical vapor deposition (CVD) of transition metal dichalcogenides (TMDs) via van der Waals epitaxy on epitaxial graphene (EG) leads to rotationally commensurate TMDs in contrast to randomly aligned TMDs grown on amorphous oxide substrates. However, the interlayer coupling between TMDs and EG hinders the investigation and utilization of the intrinsic electronic properties of the resulting TMDs, thus requiring their isolation from the EG growth substrate. To address this issue, we report here a technique for selectively transferring monolayer molybdenum disulfide (MoS2) from CVD-grown MoS2-EG van der Waals heterojunctions using copper (Cu) adhesion layers. The choice of Cu as the adhesion layer is motivated by density functional theory calculations that predict the preferential binding of monolayer MoS2 to Cu in contrast to graphene. Atomic force microscopy and optical spectroscopy confirm the large-scale transfer of rotationally commensurate MoS2 onto SiO2/Si substrates without cracks, wrinkles, or residues. Furthermore, the transferred MoS2 shows high performance in field-effect transistors with mobilities up to 30 cm2/Vs and on/off ratios up to 106 at room temperature. This transfer technique can likely be generalized to other TMDs and related 2D materials grown on EG, thus offering a broad range of benefits in nanoelectronic, optoelectronic, and photonic applications.
关键词： molybdenum disulfide,van der Waals epitaxy,two-dimensional semiconductors,field-effect transistors,copper adhesion layers,transition metal dichalcogenides,chemical vapor deposition
更新于2025-09-23 15:21:21
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Graphene-assisted molecular beam epitaxy of AlN for AlGaN deep-ultraviolet light-emitting diodes
摘要： We report on the van der Waals epitaxy of high-quality single-crystalline AlN and the demonstration of AlGaN tunnel junction deep-ultraviolet light-emitting diodes directly on graphene, which were achieved by using plasma-assisted molecular beam epitaxy. It is observed that the substrate/template beneath graphene plays a critical role in governing the initial AlN nucleation. In situ reflection high energy electron diffraction and detailed scanning transmission electron microscopy studies confirm the epitaxial registry of the AlN epilayer with the underlying template. Detailed studies further suggest that the large-scale parallel epitaxial relationship for the AlN epilayer grown on graphene with the underlying template is driven by the strong surface electrostatic potential of AlN. The realization of high-quality AlN by van der Waals epitaxy is further confirmed through the demonstration of AlGaN deep-ultraviolet light-emitting diodes operating at 260 nm, which exhibit a maximum external quantum efficiency of 4% for an unpackaged device. This work provides a viable path for the van der Waals epitaxy of ultra-wide bandgap semiconductors, providing a path to achieve high performance deep-ultraviolet photonic and optoelectronic devices that were previously difficult.
关键词： AlGaN,AlN,molecular beam epitaxy,deep-ultraviolet,light-emitting diodes,van der Waals epitaxy
更新于2025-09-23 15:21:01
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Epitaxial Growth of Monolayer MoS <sub/>2</sub> on SrTiO <sub/>3</sub> Single Crystal Substrates for Applications in Nanoelectronics
摘要： Monolayer molybdenum disulfide (MoS2) crystals grown on amorphous substrates such as SiO2 are randomly oriented. However, when MoS2 is grown on crystalline substrates, the crystal shapes and orientations are also influenced by their epitaxial interaction with the substrate. In this paper we present the results from chemical vapor deposition growth of MoS2 on three different terminations of single crystal strontium titanate (SrTiO3) substrates. On SrTiO3(111) the monolayer MoS2 crystals form equilateral triangles with two main orientations, in which they align their <21?1?0>-type directions (i.e., the sulfur-terminated edge directions) with the <11?0>-type directions on SrTiO3. This arrangement allows near perfect coincidence epitaxy between seven MoS2 unit cells and four SrTiO3 unit cells. On SrTiO3(110) the MoS2 crystals tend to align their edges with both <11?0> and <11?2?> directions on SrTiO3 as these both provide favorable coincidence lattice registry. This distorts the crystal shapes and introduces an additional strain detectable by photoluminescence. When triangular MoS2 crystals are grown on SrTiO3(001), they again show a preference to align their edges with the <11?0> directions on SrTiO3. Our observations can be explained if the interfacial van der Waals (vdW) bonding between MoS2 monolayers and SrTiO3 is greatest when the maximum commensuration between the lattices is achieved. Therefore, a key finding of this paper is that the vdW interaction between MoS2 and SrTiO3 substrates determines the supported crystal shapes and orientations by the epitaxial relations. Controlled crystal orientations make the growth of large sheets of MoS2 possible when there are multiple nucleation sites. This minimizes the number of grain boundaries and optimizes electronic properties of the material, e.g., charge mobility, which is crucial for the application of monolayer MoS2 in next-generation nanoelectronic devices.
关键词： Raman spectroscopy,van der Waals epitaxy,scanning tunneling microscopy,SrTiO3,2D materials,chemical vapor deposition,MoS2,photoluminescence spectroscopy
更新于2025-09-23 15:21:01
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van der Waals epitaxial growth of single crystal α-MoO3 layers on layered materials growth templates
摘要： Monolayer and multilayer α-MoO3 nanosheets are successfully grown on a 2D substrate by simply evaporating amorphous molybdenum oxide thin film in ambient conditions. A single-crystal α-MoO3 nanosheet without grain boundary is epitaxially grown on various 2D substrates despite a large lattice mismatch. During growth, the quasi-stable monolayer α-MoO3 first covers the 2D substrate, then additional layers are continuously grown on the first monolayer α-MoO3. The band gap of the α-MoO3 increases from 2.9 to 3.2 eV as the thickness decreases. Furthermore, due to oxygen vacancies and surface adsorbates, the synthesized α-MoO3 is highly n-doped with a small work function. Therefore, α-MoO3 field-effect transistors (FETs) exhibit a typical n-type conductance. This work shows the great potential of ultra-thin α-MoO3 in 2D-material-based electronics.
关键词： work function,van der Waals epitaxy,band structure,vacancy,transition metal oxide,stability,2D materials,lattice mismatch,transistor,heterostructure
更新于2025-09-23 15:21:01
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van der Waals Epitaxial Growth of Atomically Thin 2D Metals on Dangling-Bond-Free WSe <sub/>2</sub> and WS <sub/>2</sub>
摘要： 2D metals have attracted considerable recent attention for their special physical properties, such as charge density waves, magnetism, and superconductivity. However, despite some recent efforts, the synthesis of ultrathin 2D metals nanosheets down to monolayer thickness remains a significant challenge. Herein, by using atomically flat 2D WSe2 or WS2 as the growth substrate, the synthesis of atomically thin 2D metallic MTe2 (M = V, Nb, Ta) single crystals with the thickness down to the monolayer regime and the creation of atomically thin MTe2/WSe2 (WS2) vertical heterojunctions is reported. Comparison with the growth on the SiO2/Si substrate under the same conditions reveals that the utilization of the dangling-bond-free WSe2 or WS2 as the van der Waals epitaxy substrates is crucial for the successful realization of atomically thin MTe2 (M = V, Nb, Ta) nanosheets. It is further shown that the epitaxial grown 2D metals can function as van der Waals contacts for 2D semiconductors with little interface damage and improved electronic performance. This study defines a robust van der Waals epitaxy pathway to ultrathin 2D metals, which is essential for fundamental studies and potential technological applications of this new class of materials at the 2D limit.
关键词： van der Waals epitaxy,field-effect transistors,2D materials,chemical vapor deposition,transition metal dichalcogenides
更新于2025-09-19 17:15:36
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Optical properties of GaN Nanowires grown on chemical vapor deposited-Graphene
摘要： Optical properties of GaN nanowires grown on chemical vapor deposited graphene transferred on an amorphous support are reported. The growth temperature was optimized to achieve a high nanowire density with a perfect selectivity with respect to a SiO2 surface. The growth temperature window was found to be rather narrow (815 ± 5) °C. Steady-state and time-resolved photoluminescence from GaN nanowires grown on graphene was compared with the results for GaN nanowires grown on conventional substrates within the same molecular beam epitaxy (MBE) reactor showing a comparable optical quality for different substrates. Growth at temperatures above 820°C led to a strong NW density reduction accompanied with a diameter narrowing. This morphology change leads to a spectral blueshift of the donor bound exciton emission line due to either surface stress or dielectric confinement. Graphene multi-layered micro-domains were explored as a way to arrange GaN nanowires in a hollow hexagonal pattern. The nanowires grown on these domains show a luminescence spectral linewidth as low as 0.28 meV (close to the set-up resolution limit).
关键词： van der Waals epitaxy,photoluminescence,nanowires,GaN growth on graphene,graphene micro-domains
更新于2025-09-19 17:15:36
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van der Waals Epitaxy of Earth-Abundant Zn <sub/>3</sub> P <sub/>2</sub> on Graphene for Photovoltaics
摘要： Earth-abundant semiconducting materials are a potential solution for large-scale deployment of solar cells at a lower cost. Zinc phosphide (Zn3P2) is one such earth-abundant material with optoelectronic properties suitable for photovoltaics. Herein, we report the van der Waals epitaxy of tetragonal Zn3P2 (α-Zn3P2) on graphene using molecular beam epitaxy (MBE). The growth on graphene progresses by the formation of Zn3P2 triangular flakes, which merge to form a thin film with a strong (101) crystallographic texture. Photoluminescence (PL) from the Zn3P2 thin films is consistent with previously reported Zn3P2. This work demonstrates that the need of a lattice-matched substrate can be circumvented by the use of graphene as a substrate. Moreover, the synthesis of high-quality Zn3P2 flakes and films on graphene brings new material choices for low-cost photovoltaic applications.
关键词： Zn3P2,graphene,molecular beam epitaxy,photovoltaics,Van der Waals epitaxy
更新于2025-09-19 17:13:59
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Pulsed Laser Deposition Assisted van der Waals Epitaxial Large Area Quasi‐2D ZnO Single‐Crystal Plates on Fluorophlogopite Mica
摘要： There are still challenges in growth of transferable large area orientated ultrathin high-melting-point metal oxide single crystals with conventional methods. Herein, a new pathway to produce high quality single-crystal ZnO nanoplates with more than 400 μm crystal size is revealed by using the van der Waals epitaxy (vdWE) combined with pulsed laser deposition (PLD) method on fluorophlogopite mica. The quasi-2D ZnO plates as thin as 5 nm on fluorophlogopite mica without transition layer are achieved, showing an excellent thickness and orientation control while maintaining the excellent crystalline. ZnO nanoplates grown on conducting graphite and insulating hexagonal boron nitride (h-BN) 2D substrates are also obtained through PLD assisted vdWE. The transfer of 15 nm thick quasi-2D ZnO plates with 8 mm × 8 mm area onto a SiO2/Si substrate is successfully demonstrated. Based on the ZnO nanoplates, semitransparent self-powered ultraviolet (UV) photodetectors and light-emitting diodes centered at 400 nm UV region are demonstrated. This research highlights that the PLD assisted vdWE method is a fascinating way to fabricate high coverage ultrathin 2D ZnO plates with precisely thickness control for optoelectronic applications and may have enormous inspiration for other 2D nanomaterials growth.
关键词： pulsed laser deposition,van der Waals epitaxy,quasi-2D,ZnO single-crystal plate
更新于2025-09-19 17:13:59
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Ultra-fast and high flexibility near-infrared photodetectors based on Bi2Se3 nanobelts grown via catalyst-free van der Waals epitaxy
摘要： van der Waals epitaxy (vdWe) method has recently attracted considerable interest due to its extensive application in the growth of layered structure materials. However, the growth process of vdWe is not completely understood. Here, we report the controlled growth process of Bi2Se3 nanobelts and study their photoresponse behaviour. The average length of nanobelts increases from 7 mm to 17 mm by adjusting the temperature of Bi2Se3 powder from 520 (cid:1)C to 530 (cid:1)C, however the average length becomes saturated with further increasing the source material temperature over 530 (cid:1)C. Such a change can be attributed to the competition between the process of Bi2Se3 molecule diffusion and the process of crystal formation-related chemical reaction, leading to a symmetrically studying for the growth process of catalyst-free vdWe growth of Bi2Se3 nanobelts. The photodetectors based on these Bi2Se3 nanobelts show excellent device performance in the near-infrared light range, including an ultra-fast photoresponse (trsing z 37 ms, tdecay z 62 ms), a high responsivity of 10.1 mA/W and a high detectivity of 4.63 (cid:3) 108 Jones. This high device performance could be related to the excellent carrier transport properties as Bi2Se3 nanobelt photodetectors also demonstrate a great potential for fabricating ?exible and wearable electronics by still showing stable photoresponse after bending the device for 200 times.
关键词： 2D NIR photodetectors,Catalyst-free van der Waals epitaxy,Bi2Se3 nanobelts
更新于2025-09-12 10:27:22
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Salt‐Assisted Growth of P‐type Cu <sub/>9</sub> S <sub/>5</sub> Nanoflakes for P‐N Heterojunction Photodetectors with High Responsivity
摘要： P-n junctions based on two dimensional (2D) van der Waals (vdW) heterostructure are one of the most promising alternatives in next-generation electronics and optoelectronics. By choosing different 2D transition metal dichalcogenides (TMDCs), the p-n junctions have tailored energy band alignments and exhibit superior performance as photodetectors. The p-n diodes working at reverse bias commonly have high detectivity due to suppressed dark current but suffer from low responsivity resulting from small quantum efficiency. Greater build-in electric field in the depletion layer can improve the quantum efficiency by reducing recombination of charge carriers. Herein, Cu9S5, a novel p-type semiconductor with direct bandgap and high optical absorption coefficient, is synthesized by salt-assisted chemical vapor deposition (CVD) method. The high density of holes in Cu9S5 endows the constructed p-n junction, Cu9S5/MoS2, with strong build-in electric field according to Anderson heterojunction model. Consequently, the Cu9S5/MoS2 p-n heterojunction has low dark current at reverse bias and high photoresponse under illumination due to the efficient charge separation. The Cu9S5/MoS2 photodetector exhibits good photodetectivity of 1.6 × 1012 Jones and photoresponsivity of 76 A W?1 under illumination. This study demonstrates Cu9S5 as a promising p-type semiconductor for high-performance p-n heterojunction diodes.
关键词： Cu9S5,2D metal chalcogenides,photodetectors,van der Waals epitaxy,p-n heterojunctions
更新于2025-09-12 10:27:22